The OptimizePress video player will also support MP3 audio files.
You need to host your mp3 file either on your own website hosting or a service like Amazon S3. Once you have uploaded your MP3 file you would need to use the on page video settings. In the field where you would normally add a video URL, you would instead add your audio mp3 file URL.
Please note you will need to also set the audio player settings to a suitable height - .e.g 50 pixels for the height of the player and a suitable width also.